The discharge of produced water in the Norwegian Sector of the North Sea is expected to increase from 15 Mill m.sup.3 (1991) to 110 Mill m.sup.3 by year 2000.sub.(1).
Liquid--liquid hydrocyclones are proven successful in handling increasing water production and in maintaining the current discharge limits of 40 ppm for dispersed aliphatic hydrocarbons. The average discharge concentration of dispersed oil in the Norwegian Sector has been relatively constant over the last years at approximately 20 ppm(1). However, as the existing reservoirs will be operated at higher water cuts by the year 2000, the water treatment capacity is expected to be the bottleneck in maintaining the oil production capacity for many fields in the North Sea.sub.(1).
There is a growing concern over the amount of aromatic compounds in the water phase, such as Benzene, Toluene and Xylene (BTX), naphtalene and PAH due to the toxic effect on the marine environment.sub.(2). Though no restrictions or limits to the discharge of aromatic compounds exists, it is anticipated that, when a feasible technology for its removal emerge, maximum discharge limits for aromatics will presumably follow.
In order to counter the water treatment system bottleneck, extensive research has been conducted into the improvement of the efficiency of the hydrocyclones. It has only lead to marginal improvements, some 30-40%, over the original design as proposed by Coleman and Thew in 1980.sub.(3). The major improvement in overall separation efficiency, has primarily resulted from process optimization upstream the hydrocyclones. The elimination of turbulent flow regimes generated by pumps and valves, have reduced oil drop break-up.sub.(4) and consequently improved the (downstream) separation efficiency of the hydrocyclones. The principal components governing the separation efficiency of hydrocyclones are the density difference between the continuous phase (water) and the dispersed phase (oil) and the droplet (particle) size.
When a hydrocyclone is operated at its optimum flow rate and pressure-drop, the separation efficiency can only be improved by increasing the density difference between the two phases and by minimizing droplet break-up. At normal operating conditions the density difference is given by the inherent properties of water and oil. Minimizing droplet break-up, by restricting exposure of the fluids to turbulent flow regimes, becomes a precondition for good hydrocyclone performance. This is normally achieved by housing the hydrocyclone(s) within a pressure vessel with the feed lines submerged in the liquid.sub.(5). At off-shore installations it is preferable to operate the hydrocyclone as close to the well head pressure as possible. This provides feed pressure, and it should be positioned upstream the level control valve of the Three Phase Separator to minimize droplet break-up, as illustrated in FIG. 1A.sub.(5).
Hydrocyclones is the system of choice when opened at design capacity in conjunction with a complementary flotation process at the degasser. It is, however, apparent from published data that hydrocyclones barely meet the current oil discharge limit of 40 ppm without incorporation of the downstream flotation process as provided by the degasser.sub.(5).
At increasing water cuts, oil production rates are dictated by the water treatment capacity of hydrocyclones and compounded negative separation effect created by reduced residence times in the 1st Stage Separator and in the degassor. Expanding current process capacities is often cost prohibitively expensive because of weight and space limitations, since the whole process train from the 1st Stage Separator to the degassor (flotation) equipment, must be adapted.
There is an apparent evolving need for technologies to increase the produced water treatment capacity and efficiency within the weight and space constraints of existing production platforms.